Recent years have seen the advent of smaller and higher-performance electronics and increasing efforts to produce even smaller semiconductor devices having more terminals with a finer pitch. A result is rapidly increasing numbers of semiconductor chips (devices) packaged using a tape carrier (tape carrier package; hereinafter, “TCP”) or flip chip bonded directly onto a substrate.
These types of packaging require a structure including connection bumps disposed on electrode pads of the semiconductor device, so as to provide connection terminals to the semiconductor device for electrical connection to external wires. Typically, a protection film is formed on the electrode pads to protect the electrode pads, and the bumps are formed on the electrode pads in openings where the protection film is removed.
Bumps are formed on an industrial scale, for example, by an electroplating process whereby Au (gold) or solder bumps are formed by electroplating or by a ball bump process whereby Au or solder balls are bonded onto pads by ultrasound.
Electroplating processes are advantageous in achieving large numbers of terminals and a fine pitch. However, problems arise where manufacturing equipment, such as a sputtering device and a photo device, is needed apart from an electroplating device, because the processes require formation of a barrier metal layer which doubles as a conductive film in electroplating and of windows for bump formation by the coating, exposure, and development of a photoresist.
Ball bump processes basically need no more manufacturing equipment than a wire bonder. However, the pad pitch has reached a limit: about 80 μm at commercial levels and about 60 μm at developmental levels. This indicates that ball bump processes are disadvantageous in achieving large numbers of terminals and a fine pitch.
Under these circumstances, an electroless plating bump process has emerged recently as a new bump formation process and will be soon used on a commercial basis. The electroless plating bump process is a method whereby the electrode pads of a semiconductor device are selectively electroless-plated. The following gives details of bump formation by the process.
The oxidation film and remaining thin film are removed from the electrode pads. Then, the electrode pads is subjected to a zincate process to replace the Al (aluminum) on the surface of the electrode pads with Zn (zinc). The electrode pads may be subjected to a palladium activation process, instead of a zincate process, to let Pd (palladium) to adhere to the surface of the electrode pads.
Next, the electrode pads are immersed in an electroless Ni (nickel) plating solution to set off an “Ni electroless plating reaction” (details follows). Ni reacts with and replaces the Zn or Pd on the surface of the electrode pads, and precipitates on the surface of the electrode pads. Then, the precipitated Ni itself acts as a catalyst (autocatalytic reaction) so that more Ni precipitates.
After the completion of Ni plating, to protect the Ni surface from oxidation, the surface is subjected to Au immersion plating to precipitate Au on the surface.
If the bumps are formed by the electroless plating bump process in this manner, no conductive film for use in plating needs to be formed using a sputtering device. No windows need to be formed in the photoresist above the bump formation sites using a photo device. Thus, investment in equipment is advantageously reduced in comparison to an electroplating process. Besides, the process utilizes a cheap major material (Ni) and exhibits a good throughput, and results in lower manufacture cost than an electroplating process forming Au bumps.
Bump formation by an electroless plating process is disclosed in, among others, Japanese Unexamined Patent Application 63-164343/1988 (Tokukaisho 63-164343, published on 7 Jul. 1988), Japanese Unexamined Patent Application 63-305532/1988 (Tokukaisho 63-305532, published on 13 Dec. 1988), Japanese Unexamined Patent Application 3-209725/1991 (Tokukaihei 3-209725, published on 12 Sep. 1991), Japanese Unexamined Patent Application 5-47768/1993 (Tokukaihei 5-47768, published on 26 Feb. 1993), and Japanese Unexamined Patent Application 8-264541/1996 (Tokukaihei 8-264541, published on 11 Oct. 1996).
Problems do exist however in a conventional arrangement of connection terminals where bumps are formed by an electroless plating process: bumps cannot made higher to allow for a finer pitch and more terminals.
Properties of electroless plating are the causes. That is, electroless plating does not open windows in the photoresist, and plating progresses horizontally too after plating becomes higher than the protection film. If bumps are made higher on fine-pitch chips with small spaces between pads, adjacent bumps may touch each other and be shorted. The apprehension places a restriction on the height of the bumps.
Of course, bumps can be made higher by narrowing the opening width of the protection film and the bump width. However, if the opening width of the protection film is reduced, the bumps adhere to the electrode pads on a reduced area, and adhesion strength decreases, which is undesirable. In other words, the opening width of the protection film has a lower limit below which sufficient adhesion strength is not guaranteed between the electrode pads and the bumps. The opening width of the protection film cannot be reduced below this lower limit.